Tankers can be made to function just fine, though it takes some attention to detail. I have built 3 from the ground up(new barrels and op rods), and one with a used barrel and op rod. All function just fine. Though I've been told that using a BM59/62 gas cylinder actually makes the conversion a bit easier. I have not yet tried that.
As long as the barrel doesn't get shorter than 16 1/2 inches, and the length no shorter than 26", you should have no issues with NFA. Though I don't think I Have seen a 16 1/2" barrel. Converting an op rod to shorter length isn't that hard, and some of the old conversions h ad some really short op rods that might work.
With a standard 18 1/2 barrel, 26" comes to the end of the receiver. 30" to the end of the pistol grip, so as I stated, there should be no NFA problems.
FWIW, Sarco recently had their Cittdal made Tanker barrels on sale for $100 in both 30-06 and 308Win, but it appears that they are now sold out. Personally, I would go with the 308W in the shorter barrel.